Process for extracting acetic acid from aqueous mixtures



Sept. 9, 1941. T. o. wENTwoRTl-l 2,255,235 fROCESS FOR EXTRACTING ACETIC ACID FROM AQUEOUS MIXTURE'S I Filed July 2o, 193s ATTCRNEY.

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of aqueous Aacid in any ,extraction process Paened ses; 9.1941

UNITED STTES PATENT* o'FFicE rnooass ron t Fnolu aomious Theodore o. Wentworth, oineinhadvohio Application .my zo. leas, semi No. 220.114

s claims. (cl. cto-54n- This invention relates to a process for the separation of acetic ac id from aqueous solutions,

and the present application is a continuation in part oi.' my application nled December 16, 1936,

Serial No. 116,11() (Patent 2,123,348 of July 12, 1938) The process claimed in the said application relates to the dehydration of aqueous acetic acid by azeotropic distillation thereof in a column still, the aqueous acetic acid being introduced to a mid-point of the column, and the furfural-water-vapors distilled over the top or decanted. the iuriural being returned to the head of the columnas redux and an additional amount or furfural beine passed to the head of the column as reux above that required for the zo e tropic mixture and sumcient to hold down the acid to produce substantially acid-free water at the top of the column.

, The present invention relates to a u" f a. extraction of the aqueous acetic acid. iurfural. Furfuralhas a very favorable distribution coeiiicient, i. e., the percentage oi' acetic acid'in a furfural layer is approximately the same as the percentage of acetic acid in a water layer in contact therewith.- This is very good, considering that most acetic acid extractants hitherto used whichhave the high boiling range of iurfural usually lhave much .less favorable distribution coefficients. A

Since lurfural is heavier than water or aque- `ous acetic acid, it will usually be p countercurrently downwardly against the rising stream in contradistinction to the usual practice of solvents, having a lower speciiic gravity than water, upwardly against a descending stream of 'the column being condensed-and the condensate 'therefrom to permit its ready' separation point of the added solvent close to'that ot iuriural and in anycase, higher than that oi.' acetic acid and sufllciently divergent by ordinary distillation methods.' The quantity of such added solvent may vary from 5% to 50% of the solventiurtural mixture, dependingon the desired water content or .the furfural layer leaving the extraction process. I have found,- for example. that octyl acetate is a suitable solvent for.

addition to iuriural for the watersolubility, and that there are other organicmaterials and mixtures which have physical' 1 properties similar to octyl acetate and which,

vwhen combined with iuriural as a solventfexhibit to a greater or lesser degree the eiiectsde'- scribed above.

aqueous acid. I have found that it is desirable to carry out this liquid-liquid extraction priorto the azeotropic distillation at as low atemperature as practical, in order to reduce to a minimum the quantity of water which dissolves in the furfural layer leaving the extraction process and which contains the extracted acid.

clbla that ls, should dissolve a negligible quan tity of water, and should be completely ible with both acetic acid and furfural. The boil t,

For example, in thek handling of aqueous acid solutions of various concentrations, I have found that by the use of an emcient extractor and' using` approximately twice as much i'urfural as aqueous acetic acid, it is possible to remove all but about 0.1% acetic acid from the watery layerleaving the extractor. The furfural layer thus.

discharged may contain from 1% to 15% acetic acid, depending on the initial acid concentrationl of the aqueous solution subjected to extraction,

and from 5 to' 7% water. The exact quantity/ of water which the furfural4 layer will contain .is ixed'by the acid concentration,l the exit tem'- perature at which the iuriural layer leaves the extractor, and the 'quantity solvent, 'auch as octyl acetate, whichihas been added to the solvent fu'rf'ural for the purpose of reducing water solubility. By means of this ex-` traction step. the greater part of the water in the original aqueous solution'is eliminated. The i'urtural layer is then fed to the middle'point of the azeotropic distilling column, the operation of which has' been previously described, and the water separated from the acetic, acid and furfural therein by the azeotropic distillation. The acetic acid-iurtural mixture fromthe base of the azeotropic column is then fractionated to sepaV rate the concentrated acetic acid and furfural, which maycontain an added solvent, whichma-y terial is divided with part going to the extracting operation and part to the head of the distilling column.

The process will be described with reference to theaccompanying drawing. In the description Y which follows, reference is made to furfural only 1 as the solvent for the sake of clarity. It is understood, however, that the solvent furfural may contain an added solvent such as octyl acetate, in those instances where process conditions make the use or such added solvent desirable'. In the drawing,- the ligure is a schematic view of a complete assembly of units for eiecting theextrac -tion of theacetic acid and for recovery of the should be reasonably purpose of reducing of water-insoluble I tor in counter-current relationship to the dilute acetic acid. In -this contacting it extracts substantially all of the acid and discharges from the base of the extracting column in valved pipe line l to distiiling column 5. Distilling column operates as an azeotropic rectifying column and discharges from the top a mixture of vapors of furfural and water to condenser I, wherein they are condensed and the condensate flows to decanter 1.

In decanter 1 two layers are formed, i. e., an aqueous layer and a furfural layer. The furfural layer being heavier, discharges from the bottom and is passed to the top of the azeotropic column 5 to serve as reflux in preventing acid vapors from rising in this column.

Column 5 is suitably heated by a steam coil 8 and by the time the liquid mixture reaches the base of this column and the steam coil 8 substantially all of the water has been separated therefrom and the anhydrous mixture of acetic acid and furfural discharges by the valved line 9 to an acid rectifying column I0 wherein the acetic acid is rectified as vapors substantially free from lfurfural at the top in the normal method of rectification. The vapors are condensed in the condenser II, a part returned as reux through the line I2 and the balance discharged as the substantially anhydrous acid.

The furfural in its turn is substantially separated by the rectification action in column I0 and is discharged at the base substantially free of acid through the valved pipe line I3 and, after suitable cooling, is pumped by means of the pump I4 to the storage tank 3 for furiural. Furfural is also drawn from the storage tank 3 by means of a valved line I5 to the top of the azeotropic column 5. This furfural serves as reflux liquid in addition to that supplied from the decanter. Another stream of furfural drawn from the tank 3 by the valved pipe line I6 feeds the extractor 2 as already mentioned.

The water discharged from the acid-extracting ,column 2 bymeans of pipe line II is, of course,

saturated with furfural, as is also the Water discharged from the decanter 'I by means of the pipe extractor. This water will, of course, be saturated with the furfural extracting agent such as ethylene dichloride in this case. In the usual case, it will be possible to so select this extracting material from the relatively inexpensive solvents avaliable that it win notte worth whue uA re- .cover the small amount of solvent discharged in this water layer. In those few cases where it might be worth while, this may be done in the usual distillation manner as has been standard practice in ally work relating to extraction and azeotropic distillation 9i acetic acid.

The ethylene dichloiide used for this extraction is stored in the supply tank 23 and passes .to the extractor by the valved line 24. It then descends in the extracting column 2| and by countercurrent washing picks up all of the furfural previously dissolved in the water; it discharges at the base through the valved line 25 and passes to the distilling column 26 wherein the ethylene dichloride, being comparatively volatile, is removed at the top as vapors. condensed in the condenser 21, partly returned as reflux to the column 26 by means of the valved line 28 and partly discharged to the ethylene dichloride storage tank 23. Any small amount of water which may be dissolved in the ethylene dichloride-furfural mixture will o! course, be distilled with the ethylene dichloride but will make no difierence in the operation of the system, since the water may be readily decanted 0H. From the base of the column 26 the furfural is discharged substantially pure by means o! the valved line 28 and, after suitablev covering furfural from the aqueous layers disline I8 and both streams are discharged to thev water storage tank I9. WaterI in the tank I0 must'I be suitably treated to remove the furfural for re-u'se. The usualmethod for effecting this recovery of furfural would be by means of `steam distillation.

In the present case the water saturated with iurural yis passed bymeans of the valved line 2li to the furfural extractor 2|. In this extractor the furfural-saturated water is counter-currently contacted with a stream of suitable organic liquid Vsuch as paraffin hydrocarbons, aromatic hydrocarbons, or their halogenated derivatives. In the present case, a material such as ethylene dichloride is utilized which has a higher speciiic gravity than Water saturated with furfural and thus will descend in the extraction column. In this extracting operation the water flows upwardly in' 2I and is entirely freed of furfural, thendischarging by means of the pipe line 22 from this charged from the acid extractor and the decanter by means of. a subsequent liquid extraction with ethylene dichloride or with one or a mixture of hydrocarbons selected from the group of paraflln hydrocarbons, aromatichydrocarbons, their chlorinated derivatives. Also to be noted is the fact that in some cases the liquid or liquid mixture used for removing the furfural and the furfural itself may be heavier or lighter thanwater, depending on the selection of the material and the ratio of this material to furfural which-may be used. It is, of course, necessary to use the liquid-liquid extractor in such a way that two liquid layers are formed separating in opposing currents.

Having described my invention, what I claim is:

1. The step in the dehydration of aqueous acetic acidwhich comprises subjecting the same to a counter-current of furfural, containing an added organic solvent other than furfural but having a boiling point in the range of furi'ural and.

which will dissolve a negligible quantity of water and which is completely miscible with acetic acid of aqueous acetic acid to a counter-current ilow of substantially twice the volume of furfural, in

van extractor.

THEODORE o. WEN'rwoRTH. 

